1. Field of the Invention
The present invention relates to an oil-in-water type cosmetic composition containing retinoids. More particularly it relates to an oil-in-water type cosmetic composition containing retinoids stabilized within a core of a liquidcrystal formed by a surfactant having a high phase transition temperature and a bulky structure.
2. Description of Related Art
In a cosmetic field, there have been many studies for development of new material which can promote cell differentiation and regeneration, and can increase collagen biosynthesis, and is safe to the skin. As a material satisfying these needs, vitamins have been concentrated on and utilized as a cosmetic ingredient. Among these vitamins, retinoids are very effective in improving a skin state, and therefore many concerns have been concentrated on these retinoids.
It is known that retinoids are very effective in recovering the skin from various damages caused by wrinkles, roughness, drying or abnormal peeling(U.S. Pat. No. 4,603,146 and U.S. Pat. No. 4,877,805). Retinoids include retinol, retinal, retinyl palmitate, retinyl acetate, retinoic acid and the like. Among them, in consideration of efficacy and safety to the skin, retinol is the most compatible for the skin.
Retinol and its derivatives have an outstanding effect in preventing the formation of skin wrinkles. However, they are easily oxidized in an air or in an aqueous solution, and loss their stability and physiological activities. Due to their instability, there has been a limit in incorporating retinol into cosmetic formulations.
Under this circumstance, many studies have been conducted to stabilize retinol in a cosmetic composition. In particular, there has been proposed an oil-in-water type(O/W type) emulsion wherein retinol is stabilized by an antioxidant such as BHT(butylated hydroxytoluene), BHA(butylated hydroxyanisole), tocopherol and its derivatives, ascorbic acid(vitamin C) and citric acid. For examples, U.S. Pat. No. 4,466,805 discloses an O/W type cosmetic composition containing retinol stabilized by an antioxidant such as BHT and dl-.alpha.-tocopherol and a chelating reagent such as EDTA(ethylenediaminetetraacetic acid). And, U.S. Pat. No. 4,247,547 discloses a gel-type cosmetic composition containing retiol stabilized by an antioxidant such as BHT, BHA, ascorbic acid, propyl gallate and .alpha.-tocopherol.
However, although retinoids can be stabilized to some extent by these antioxidants, excessive use of antioxidant may cause skin irritation. And, there is a limit in stabilizing retinoids without blocking contact with water which is present in a cosmetic base. Further, since instability of retinoids may be accelerated by oxygen attack through the medium of aqueous solution, many research for stabilization of retinoids are being conducted in a structural aspect of emulsion system itself.
On the other hand, in water-in-oil type(W/O type) emulsion, since water of a core is surrounded with outer oily film and oxygen supply from outer atmosphere is blocked, retinoids can be more stabilized than in an O/W type emulsion. Further, U.S. Pat. No. 4,826,828 discloses a W/O type emulsion containing retinoids together with a fat-soluble antioxidant such as BHT and BHA. However, W/O type emulsion tends to be oily and greasy, resulting in a poor feel of cosmetics. Additionally, it is difficult to stabilize an emulsion itself
Accordingly, the present inventors have conducted extensive studies in order to stabilize retinoids in an O/W type emulsion rendering good feel to the skin.
In order to stabilize retinoids, above all, a contact with air or water has to be blocked to a minimum as possible. Accordingly, the present inventors made efforts to find a method for blocking retinoids in a core from being in contact with water in an outer and for minimizing the transfer of components between these compartments.
In general, a combination of surfactants, fatty alcohols and polyols forms liquidcrystals. However, since this liquidcrystal has flexibility and permeability, material transfer between a core and an outer may occur through liquidcrystal film, and thereby it is difficult to stabilize retinoids of a core. The present inventors found that use of surfactants having a high phase transition temperature and a bulky structure forms a strip-shape, wide and dense liquidcrystal film and thereby material transfer between a core and an outer can be remarkably reduced. They guessed active ingredients of a core to be trapped and fixed within a lamella structure of a thick and dense liquidcrystal film. The present invention was accomplished based on this finding.